A typical acoustic imaging transducer consists of an array of piezoelectric elements disposed on a planar surface for radiating an acoustic beam in a direction generally normal to that surface and for receiving reflecting pulses from a target in the patient. The elements vibrate in several modes while producing acoustic signals. The preferred mode for producing the desired beam is referred to herein as the thickness mode. By selectively phasing transmitter elements, the beam can be focused at a predetermined distance and scanned azimuthally.
The elements, being bilateral in function, generate two fundamental beams in diametrically opposed directions. In general, for good depth resolution, one of the beams has to be absorbed and that energy has to be dissipated using some form of an acoustic absorber. Conversely, the effectiveness of the transmitted beam is maximized by controlling its direction and suppressing or cancelling spurious emissions produced by undesirable modes of vibration in each of the elements.
One mode of undesirable vibration not well understood until now may be described by analogy of one of the elements to a mass/spring harmonic oscillator. This mode of vibration is called the mass/spring mode hereinafter. The mass/spring mode is compared with the desired thickness mode in FIG. 1(a) and 1(b). In the model, the backing is analogous to the "spring", or energy storage mechanism, while the PZT element bonded to the backing is the "mass". This model assumes the kinetic energy is all contained in the "mass", while all the elastic energy is stored in the "spring", i.e., negligible motion of the backing. This model further assumes rigid body motion rather than wave motion, which is ordinarily obtained in high frequency acoustic devices, and that the element is tall and narrow to obtain the relative frequencies shown.
The backing represents a relatively solid, unmoving foundation for the element as compared to the negligible loading on the element's top surface. Therefore, a voltage applied to the PZT will cause the top surface of the element to move while the bottom surface will remain relatively stationary. Thus, in the proposed model, there is net displacement of the element and excitation of the mass/spring mode.
Another undesirable mode of vibration of elements in the transducer, referred to as the dilatational mode, is also illustrated in FIG. 1(a) and compared with the other modes mentioned in FIG. 1(b). Dilatation refers to the particle motion being primarily transverse to the element. The relative frequency responses of the various modes are also illustrated in FIG. 1(a).
A piezoelectric plate, from which the elements of an imaging transducer array are formed, will expand if a voltage is applied one way with respect to the poling direction, and will contract if the voltage is reversed. In either case, the center of mass will move with respect to the backing top surface, and the mass/spring mode will be excited. In the preferred embodiment of the present invention, two such plates are bonded together with their respective poling vectors appropriately directed. As appropriately polarized voltages are applied to the plates, one plate expands while the other contracts. If the expansion of one plate is approximately equal to the contraction of the other, center of mass motion and, hence, the mass/spring mode is suppressed.